Visible-Light-Induced Photocatalyst Based on Cobalt-Doped ZincFerrite NanocrystalsGuoli Fan,Ji Tong,and Feng Li*,State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 98, Beijing 100029, China*S Supporting InformationABSTRACT: The present work reportedthe synthesis of visible-light-inducedcobalt-dopedzinc ferrite (Zn1xCoxFe2O4)photocatalystsviaafacilereductionoxidationroute, whichinvolvedrapidreductionofFe3+andCo2+cationsincolloidmillreactor, followedbyoxidationof ironandcobalt nuclei andstructural transformationunder hydrothermal conditions. Thestructural and optical properties of materials were characterized by powder X-ray diraction (XRD), scanning electronmicroscopy(SEM), transmissionelectronmicroscopy(TEM), X-rayphotoelectronspectroscopy(XPS), andUVvisdiusereectance spectra. The results indicated that metallic Co and Fe nuclei could be obtained by reduction in the colloid mill, andZn1xCoxFe2O4 nanocrystals with uniform size were successfully achieved. Furthermore, compared to ZnFe2O4, Zn1xCoxFe2O4samples (x = 0.03, 0.1, 0.2) exhibited enhanced photocatalytic activity in the degradation of methylene blue under visible lightirradiation. Especially, theZn0.8Co0.2Fe2O4 samplehad thehighestphotocatalyticactivity,whichwas mainly attributableto itssmaller bandedge. Sinceas-synthesizedZnCoferritesnanocrystalshavetheadvantagesof intrinsicchemical stabilityandmagneticproperty, itcanbeexpectedthattheymayhavepotential applicationintheeldofindustrial photodegradationoforganic pollutants.1. INTRODUCTIONIn the past two decades, much attention was paid to thephotocatalytic degradation of organic pollutants with semi-conductor compounds like TiO2 and ZnO for the remediationofhazardouswastesandcontaminatedgroundwater,13whichis becoming one of the most promising green chemistrytechnologies. Especially, visible-light-induced photocatalysts areattracting considerable interest because of their high eciencyin utilizing solar energy. TiO2 and ZnO, however, exhibit littlephotocatalytic activity under visible light irradiation, due totheir relatively large band gap of about 3.2 eV. Manymodications were carried out to extend the wavelengthrange of these UV-active oxides into the visible range.46Additionally, other visible-light sensitive photocatalysts, such asCdS, WO3and Fe2O3,79are explored; but their lowphotocatalyticactivitieslimitpractical application. Asaresult,it has always beena hot issue todevelopnewvisible-light-induced photocatalysts with enhanced activity.Interest intransition-metal ferrite nanocrystals has greatlyincreasedbecause of their importance inunderstanding thefundamentals in nanomagnetismand their extensive use inhigh-density data storage, ferrouid technology, magnetocaloricrefrigeration, magneticresonanceimaging, andheterogeneouscatalysis.1015Among them, zinc ferrite semiconductor materialwith a direct bulk band gap of 1.9 eV, is attracting considerableattention due to its potential applications as magneticmaterials,16,17semiconductor photocatalysts,17,18and gassensors.19More importantly, visible-light-derived photocatalyticperformance of ZnFe2O4has been extensively aroused.20,21However, thepoorquantumeciencyof ZnFe2O4resultsinlowphotocatalyticactivity. Inordertosolvetheproblem, themodicationhas alsobeenattemptedbyloadingAgontheZnFe2O4 surface.22On the other hand, various physicochemicalmethods to control the synthesis of uniformZnFe2O4nanoparticleshavebeenextensivelyinvestigated.2335Inmostcases, they require handling of a large amount of organic salts,toxicsolvents, orsurfactants, whichcausesexpensivecostsaswell as environment pollution. It is imperative to develop green,cheap, facile, andfast methods for thepreparationof single-phase ferrite nanoparticles. Recently, we developed a facilepathway for the synthesis of uniform ZnFe2O4 nanocrystals viaa reductionoxidation route,19which involved easily achievablehigh-speednucleationof metallicprecursorsinacolloidmillreactor, followed by oxidation of iron nuclei in a separatehydrothermal process. The applied approach oers uniqueadvantages suchas lowcost, rapidness, homogeneity, betterreproducibility, energy saving, benignancy to environment, andsimplicity,compared to other conventional methods.Considering that the sensitivity to visible light for zinc ferritemakesitpossibletoenhancethephotocatalyticactivityundervisible light through proper modulation of structure, herein, inthe present work, we prepared visible-light-induced cobalt-dopedzincferrites(Zn1xCoxFe2O4, x=0, 0.03, 0.1, 0.2, and0.4) via a reductionoxidation route. The photocatalyticactivity of Zn1xCoxFe2O4was tested in the degradation ofmethylene blue under visible light irradiation, andZn0.8Co0.2Fe2O4exhibitedthe highest photocatalytic activity.Furthermore, as-synthesized photocatalysts combined theadvantages of intrinsic chemical stability and magnetic property.To the best of our knowledge, there is no report about visible-Received: August 29,2011Revised: August 28,2012Accepted: September 27,2012Published: September 27,2012Articlepubs.acs.org/IECR 2012 American Chemical Society 13639 dx.doi.org/10.1021/ie201933g | Ind. Eng. Chem. Res. 2012,51, 1363913647light-induced photocatalyst based on ZnCo ferrite nanocryst-als until now.2. EXPERIMENTAL SECTION2.1. Synthesis of ZnCo Ferrites. In a typical procedure,twoseparatesolutions werepreparedprior tothesynthesis.Sol uti on A: anal yti cal -grade Zn(NO3)2 6H2O, Fe-(NO3)39H2O, and Co(NO3)26H2Owere dissolved in 50mL of deionized water to give a solution (x = [Co2+]/([Zn2+]+[Co2+]) = 0, 0.03, 0.1, 0.2, and 0.4; [Zn2+]+[Co2+] =1/2[Fe3+] = 0.1 M). Solution B: sodiumborohydride wasdissolved in 50 mL of deionized water to form a solution withthe [NaBH4]/([Fe3+]+[Co2+])molar ratio of2.0.Solutions Aand B were simultaneously added rapidly to a colloid mill withrotor speed set at around 6000 rpm and mixed for 2 min. Themixture slurry was sealed in a Teon-lined autoclave and heatedat120Cfor12h, respectively. Thesuspensionwaswashedwith deionized water several times and then ethanol, separately.The obtained solids were dried at 70 C for 12 h.2.2. Characterization. Powder X-ray diraction (XRD)patterns of samples were collected using a Shimadzu XRD-6000diractometerunderthefollowingconditions:40kV, 30mA,graphite-ltered CuK radiation). The samples, as unorientedpowders, were step-scannedinsteps of 0.04 (2) using acount time of 10 s/step. The crystallite size (Dhkl) wascalculated by Scherrer eq 136 =DKcoshkl(1)where K is the Scherrer constant (K = 0.89), is thewavelengthof theradiation(=0.15418nm), is thefullwidth of the (hkl) peak at half-maximum intensity, and is theBragg angle.Elemental analysis for metal ions in sampleswas performedusing a Shimadzu ICPS-75000 inductively coupled plasmaemission spectrometer (ICP-ES). Samples were dried at 100 Cfor 24 hprior toanalysis, andsolutions were preparedbydissolving the samples in dilute hydrochloric acid and nitric acid(1:1, V:V) for 24 h at room temperature.Scanningelectronmicroscopy(SEM)microanalysesof thesamplesweremadeusingaHitachi S4700apparatuswiththeapplied voltage of 20 kV, combined with X-ray energydispersive spectroscopy (XEDS) for the determination ofmetal composition.Transmission electron microscopy (TEM) images weretaken using a Hitachi H-800 transmission electron microscopyoperated at 100 kV. High resolution transmission electronmicroscopy (HRTEM) was carried on a JEM-3010 highresolutiontransmissionelectronmicroscopyat anacceleratingvoltage of 200 kV.X-ray photoelectron spectra (XPS) was recorded on aThermoVGESCALAB250X-rayphotoelectronspectrometerusing Al K X-ray as the excitation source. The binding energycalibration of the spectra has been referred to carbon 1s peak,located at BE = 284.8 eV.The specic surface area determination was performed by theBrunauerEmmettTeller (BET)methodusingaQuantach-rome Autosorb-1C-VP Analyzer.Magnetismof sampleswasmeasuredat roomtemperatureon a locally made JDM-13 vibrating sample magnetometer(VSM).Solid-state UVvis diuse reectance spectra were recordedat roomtemperature by means of a ShimadzuUV-2501PCspectrometerequippedwithanintegratingsphereattachmentusing BaSO4 as background.The decrease in total organic carbon (TOC), which indicatedthe mineralizationof MB, was determinedusing anApollo9000 TOC Analyzer (Terkmar Dohrmann Co.).2.3. Photocatalytic Reaction. The photodegradationactivities of samples were evaluated by measuring thedegradation of methylene blue (MB) in aqueous solutionundervisiblelight(>420nm). Thevisiblelightsourcewasa300WXearclamp(PLS-SXE300UV)witha420nmcutolter and I = 20 A. Typically, 100 mL of an aqueous solution ofMB(10mg/L)containing0.05gof thecatalyst samplewasultrasonically dispersed in a quartz beaker for 10 min, and thenthe suspension was vigorously agitated for 30 min in the dark toreach absorptiondesorption equilibrium before the irradiation.Subsequently, the suspension was irradiated under visible light.At the given time interval of 30 min,1 mL of suspension wascollectedtodiluteto10mLandcentrifugedtoremovethesolid residue.The concentration of the supernatant liquid wasdeterminedbyabsorbanceat 665nminUVvis absorptionspectrum. Additionally, 2,4,6-trichlorophenol andsalicylicacidsodium(10 mg/L) were used as colorless pollutants toinvestigate the photocatalytic performance of catalyst. Theconcentrationofsupernatantliquid ofMB,2,4,6-trichlorophe-nol, and salicylic acid sodium was determined by absorbance at665, 290, and 297 nm in UVvis absorption spectrum,respectively. Thepowdersample ofP25 (TiO2,Degussa)wastested as a comparison catalyst, and a blank reaction was carriedout following the same procedure without adding any catalyst.3. RESULTS AND DISCUSSION3.1. Structural Characterization of ZnCo Ferrites.Figure1 displays thepowderXRD patternsofZn1xCoxFe2O4samples. Ineachcasethecharacteristic(220), (311), (400),(422), (511), and (440) reections of simple cubic spinel phaseareclearlyobserved, andnoXRDpatternsarisingfromotherphasesappear. It conrmsthat as-synthesizedZn1xCoxFe2O4products areof highpurityandgoodcrystallinity. ElementalFigure 1. XRD patterns of Zn1xCoxFe2O4 samples: (a) x = 0; (b) x =0.03; (c) x = 0.1; (d) x = 0.2; and (e) x = 0.4.Industrial & Engineering Chemistry Research Articledx.doi.org/10.1021/ie201933g | Ind. Eng. Chem. Res. 2012,51, 1363913647 13640analysis by ICP-ES reveals that the Co/Zn atomic ratioincreases from 0 to 0.03, 0.15, 0.27, and 0.61 with increasing Cocontent, and the (Co+Zn)/Fe atomic ratios are in the range of0.480.52forallsamples(Table1). Itindicatesthattheyareindeed ZnCo ferrites with the chemical formula ofZn1xCoxFe2O4. AccordingtotheFd3mcubicspinelstructure,thevalueof latticeparameter acanbedeterminedfromthemost intense (311) reection using the formula (1/dhkl2=(h2+k2+l2)/a2). Note that the gradual introduction of Co2+ionsgive a detectable decrease in the lattice parameter a from0.8420 to 0.8341 nm (Table 1), which can be attributed to thedierence in ionic radius of Co2+(0.072 nm) and Zn2+(0.074nm). Inaddition, it isseenthat four Zn1xCoxFe2O4(x=0,0.03, 0.1, 0.2) samples display wide diraction lines, indicativeof small scattering domain sizes. The coherent diractiondomainsize, whichmaybeestimatedfromthevaluesof full-width at half-maximum (fwhm) of the (311), (511), and (440)diractions by means of the Scherrer equation, lies in the rangeof 912 nm. The Zn0.6Co0.4Fe2O4 sample, however, presents alarger domain size of about 27 nm, probably due to its highercrystallinity.SEM micrographs of samples show that three Zn1xCoxFe2O4(x = 0, 0.1, 0.2) samples present a homogeneous arrangementof aggregated nanoparticles (Figure 2a-c), and theZn0.6Co0.4Fe2O4sample is formedby larger aggregations ofparticles. TheXEDSanalysisrevealsthattheZn0.8Co0.2Fe2O4sampleiscomposedof Zn, Co, Fe, andOelements, andtheatomicratioof Zn/Co/Feisabout3.93:1:9.41, whichisveryclose to the expected stoichiometric proportion for ferrite.TEMmicrographs of Zn0.8Co0.2Fe2O4(Figure 3) furtherconrm the nature of uniform particle size. Indeed, the sampleis formedby nanoparticles, andthe average crystallite sizes(from the counting of about 100 particles) of nanoparticles areabout113nm. SincetheaveragecrystallitesizeestimatedfromTEMmicrographsandthecoherent diractiondomainsize inferred from XRD results are in good agreement,Zn0.8Co0.2Fe2O4nanoparticles aremonodispersenanocrystals.A typical HRTEM (Figure 3) image of several individual ferritenanocrystalsindicatesdierent interplanardistancesof 0.487,0.297, and 0.162 nm that are characteristic of (111), (220), and(311) spinel planes. Moreover, the selected area electrondiraction (SAED) pattern reveals the positions of discerniblediraction rings corresponding to spinel phase. The aboveresults conrmthe nanocrystalline nature of as-synthesizedZnCo ferrites.Unlike the traditional solid-state method using metal oxidesasprecursorsforsynthesisof ferrites, Zn2+/Co/Feprecursorsarealternativelyemployedinthepresent system. Here, aftermixing of Zn2+, Co2+, and Fe3+cations with a reducing agent inavigorouslystirringcolloidmill reactor, nucleationofFeandCo through the reduction of metal cations is completed withina very short time, due to energetic collision and stronghydraulic shear force in the highly turbulent liquidlm zone ofthecolloidmill. Asaconsequence, thegrowthof FeandConuclei hardly takes place due to the quite short residence timeinthe colloidmill, andthus Fe andConuclei formedtheTable 1. Composition and Parameters of Zn1xCoxFe2O4 Ferritessample ZnFe2O4Zn0.97Co0.03Fe2O4Zn0.9Co0.1Fe2O4Zn0.8Co0.2Fe2O4Zn0.6Co0.4Fe2O4Co/(Zn+Co) molar ratio,xa0 0.03 0.1 0.2 0.4Co/(Zn+Co) molar ratiob0 0.029 0.13 0.21 0.38(Co+Zn)/Fe molar ratiob0.49 0.51 0.50 0.52 0.48lattice parameter a (nm) 0.8420 0.8409 0.8373 0.8364 0.8341coherent domain size (nm) 11 9 12 11 27specic surface area (m2/g) 102 108 113 124 52magnetization (emu/g) 4 6 8 14 51k valuec(h1) 0.1735 0.2906 0.3435 0.3655 0.0832aIn synthesis mixture. bDetermined by ICP-ES. cPhotocatalytic reaction rate constant.Figure 2. SEM micrographs of Zn1xCoxFe2O4 samples: (a) x = 0; (b)x = 0.1; (c) x = 0.2; and (d) x = 0.4. Inset in (c) shows the EDS result.Figure 3. TEM (a,b) and HRTEM (c) micrographs and SEAD pattern(d) of Zn0.8Co0.2Fe2O4.Industrial & Engineering Chemistry Research Articledx.doi.org/10.1021/ie201933g | Ind. Eng. Chem. Res. 2012,51, 1363913647 13641present narrow range of particle sizes.3739The XRD pattern ofreduced intermediate collected fromthe colloid mill onlyreveals the presence of amorphous phase (Figure 4a). In orderto further reveal the synthesis process of ferrites,the obtainedintermediate is aged for 3 h at room temperature under an N2atmosphere. It isfoundthat thereducedsampleexhibitsthecharacteristicreectionsof metallicFe(JCPDS06-0696)andCo (JCPDS15-0806)phases,andno characteristicreectionscorrespondingtometallicZn(JCPDS04-0831)aredetected(Figure 4b). The aforementioned results conrma rapidnucleation of metallic Fe and Co in a colloid mill reactor. As aresult, a possible formation mechanism for the ZnCo ferritesis proposed, and the chemical reactions involved are believed toproceed as follows (eqs 26):+ + + + ++ +4Fe 3BH 12H O4Fe 3B(OH) 6H 12H34 24 2(2)+ + + + ++ +2Co BH 4H O2Co B(OH) 2H 4H24 24 2(3)+ + 4Fe 3O 6H O 4Fe(OH)2 2 3(4)+ + 2Co O 2H O 2Co(OH)2 2 2(5) + + + + + ++x xx x(1 )Zn 2Fe(OH) Co(OH)Zn Co Fe O 2(1 )H O 2(1 )Hx x23 21 2 4 2(6)In the colloid mill, Fe3+and Co2+cations are rapidly reducedtometallicFeandConuclei (eqs2and3). Then, thenewlyformedmetallicFeandConuclei areoxidizedintoFe(OH)3andCo(OH)2compounds bythetraceamount of dissolvedoxygenunderhydrothermalconditions(eqs4 and5). Finally,througha structural transformation, uniformZnCoferritesnanocrystals can be obtained (eq 6).3.2. Properties of ZnCoFerrites. The surface/near-surfacechemical states of therepresentativeZn0.8Co0.2Fe2O4sample were analyzed by XPS (Figure 5a). Core levels of Zn 2p,Co 2p, and Fe 2p can be identied. Thene spectra of Zn 2p,Co 2p, and Fe 2p peaks are presented in Figure 5b-d,respectively.The binding energy (BE) of the Zn 2p3/2 peak is1021.3 eV, indicative of the presence of the Zn2+species.TheXPS of the Co 2p region can betted into four contributions.Therst two peaks with the BE values of about 780.8 and 785.8eV are assigned to Co 2p3/2 and its shakeup satellite, while thehigher BE peaks around 796.1 and 802.5 eV correspond to Co2p1/2andits shakeupsatellite, respectively.40Inthecaseofhigh-spin Co2+-containing compounds, the BE values of intenseCo 2p shakeup satellites are about 56 eV higher than those ofFigure 4. XRD patterns for reduced immediate products in the colloidmill (a)without agingand(b)agingfor 3hat roomtemperatureunder N2 atmosphere.Figure 5. XPS spectra of Zn0.8Co0.2O4 sample: (a) survey,(b) Zn 2p,(c) Co 2p, and (d) Fe 2p.Industrial & Engineering Chemistry Research Articledx.doi.org/10.1021/ie201933g | Ind. Eng. Chem. Res. 2012,51, 1363913647 13642Co 2p transitions.41However, low-spin Co3+species onlyindicate weak Co 2pshakeup satellites. Therefore, the intenseCo2pshakeupsatellites conrmthe presence of the Co2+species inthe preparedsample. The Fe 2p3/2andFe2p1/2spectraobtainedfromthepresent studygenerallyshowtwodistinguishable main peaks with BE valuesof about 710.5and724.2 eV, respectively, accompanied by a correspondingsatellite peak for Fe 2p3/2visible at around718.7eV, onlyindicativeof thepresenceof theFe3+cations.42,43It canbefoundthattheBEvaluesof Zn2p3/2, Fe2p3/2, andFe2p1/2peaksfor Zn1xCoxFe2O4(x=0.03, 0.1, 0.2, 0.4)arealmostsimilar to those for the ZnFe2O4sample (Figure S14).Compared to that for ZnFe2O4,the intensity of Co 2p regionfor Co substituted zinc ferrites increases with the increasing Cocontent. Furthermore, with the increasing Co content, the colorof ferrites changes progressively from red brown to black.TogiveimportantinformationconcerningtheeectofCocontent ontheoptical propertiesof ZnCoferrites, UVvisdiuse reectionmeasurement was carriedout. The opticalabsorbancecanbeapproximatelycalculatedfromtheopticalreectance data by the KubelkaMunk function, = (1R)2/2R, whereistheabsorptioncoecient, andRisthediusereectance.44AsobservedinFigure6ad, withthedopingofCo content, the visible light absorption of Zn1xCoxFe2O4 (x =0, 0.03, 0.1, 0.2) is enhanced, and the absorption edge ofsampleshasasignicant redshift. Theinset showsaplot of(h)2against theenergyof absorbedlight, fromwhichthedirect allowed band gaps can be estimated. The estimated bandgapsofZnCoferritesdecreasegraduallyfrom about2.13to1.93, 1.83, and1.73eVwiththeintroductionof Co2+ions.Compared to the bulk band gap value of 1.9 eV, the absorptionedgeof as-synthesizedZnFe2O4nanocrystalshasablueshift,whichmight beascribedtothequantumconnement eectarising from the small size regime.45However, theZn0.6Co0.4Fe2O4 sample only presents a quite broad absorptionspectrumbetween400and800nm(Figure6e). Theaboveresults demonstrate that the introduction of certain amount ofCo2+ions can exhibit enhanced visible light absorption, whichprobablyoriginatesfrompropermodulationof theelectronicstructure for Zn ferrite.Theeld-dependent magnetization of ZnCo ferrite samplesat roomtemperature are shown in Figure 7. Clearly, thehysteresis varies with the dierent samples, dependent upon thecomposition of ferrites. As shown in Figure 7ad,Zn1xCoxFe2O4(x=0, 0.03, 0.1, 0.2)samplespresent room-temperature paramagnetic behavior. The value of magnetizationincreases linearly with the external magneticeld strength andcannot reach a saturation state yet, which is consistent with theearlier reports.46Withincreasing Cocontent, the measuredvalueofmagnetizationincreasesprogressivelyfrom4to6, 8,and14emu/g. Inthecasesofas-synthesizedZnCoferrites,the spinel structure should be a mixed type with Zn2+and Fe3+ionsinboththetetrahedral (A)andoctahedral (B)sitesandCo2+ions inthe preferential Bsites.4749AccordingtotheStonerWohlfarth and Ne el-Brown theories,50,51the magneticstate of the nanocrystal is determined by the magnetocrystallineanisotropy constant (K), nanocrystal volume (V), and blockingtemperature. Themagnitudeof VandKiscloselycorrelatedwith the crystal size and the strength of spinorbital couplingat crystal lattices, respectively.52Due to their small particle size,as for Zn1xCoxFe2O4(x =0, 0.03, 0.1, 0.2) samples, theirmagnetic behavior can be explained under the consideration ofsingle-domain nanoparticles. Because their ordering temper-ature is lower than roomtemperature, the superexchangeinteraction between A and B sites does not seemto befavorable at roomtemperature, hence they present para-magnetic behavior. However, the Zn0.6Co0.4Fe2O4sampleshows room-temperature ferrimagnetism with measurablecoercivity of about 32 Oe and saturation magnetization ofabout 51 emu/g. It is well-documented that the blockingtemperature becomes larger with the increasing particle size or/and the increasing anisotropy constant.49If Zn0.6Cox.4Fe2O4 isconsidered as single-domain, its ferromagnetic behaviorindicatesit presents a higher blocking temperatureand higherordering temperature than room temperature, due to both thelargerparticlesize (27 nm) and the higher magnetocrystallineanisotropy constant related to the strong spinorbital couplingat Co2+lattice sites. If this sample is considered as multidomain,Figure 6. UVvis diuse absorption spectra of Zn1xCoxFe2O4samples:(a)x=0;(b)x=0.03;(c)x=0.1;(d)x=0.2;and(e)x = 0.4. Inset shows the corresponding plots of (h)2vs the incidentphoton energy.Figure 7. Magnetization curves versus magnetic eld forZn1xCoxFe2O4samples at roomtemperature: (a) x=0; (b) x =0.03; (c) x = 0.1; (d) x = 0.2; and (e) x = 0.4.Industrial & Engineering Chemistry Research Articledx.doi.org/10.1021/ie201933g | Ind. Eng. Chem. Res. 2012,51, 1363913647 13643its higher ordering temperature thanroomtemperature canexplain its ferromagnetic behavior at room temperature.3.3. PhotocatalyticActivityof Materials. It is knownthat thephotocatalyticprocessisbasedonthegenerationofelectron/hole pairs by means of band gap radiation, which canlead to redox reactions with species adsorbed on the surface ofcatalysts. Consideringthat theCosubstitutedferritesampleshave narrower band gap energy compared with ZnFe2O4,metastable energy levels are formedwithinthe energy gap,which can shift the absorption to visible region. Therefore, Cosubstituted ferrite samples may show good photocatalyticperformance under visible light irradiation.To demonstrate the photoactivity of Zn1xCoxFe2O4 samplesfor the degradationof organic pollutants, the photocatalyticdegradationof MBexperiments was carriedout as a probereaction under visible light irradiation. The dependence of MBphotodegradationonthedierentsampleswasinvestigatedatthe same operating conditions. No obvious degradation of MBis observed in the absence of a catalyst; MB exhibits slight self-degradation under visible light irradiation.As shown in Figure8, withincreasingCocontent, the photocatalytic activity ofZn1xCoxFe2O4(x=0, 0.03, 0.1, 0.2)is enhancedgradually.The degradation percentage after 8 h over the Zn0.8Co0.2Fe2O4sample reaches 95.4%, which is much higher than that over theZnFe2O4sample. In addition, it is found that the photo-degradationprocess of MBobeys pseudo-rst-order kinetics(Figure S5). The obtained photocatalytic reaction rate constant(k) does not change monotonically with increasing Co contentinsamples (Table 1). The photocatalytic results causedbydierent ferrite samples agree well with the UVvis absorption,suggestingthat thebandgapof ferritematerialshasamajorinuenceontheir photocatalytic performance. However, thephotocatalytic activity of the Zn0.6Co0.4Fe2O4 sample is reducedmarkedly, evenlowerthanthat of theZnFe2O4sample. TheBET analysis reveals the large specic surface areas ofZn1xCoxFe2O4(x = 0, 0.03, 0.1, 0.2) samples are in therange of 102124 m2/g (Table 1). In the case of theZn0.6Co0.4Fe2O4 sample, ferrimagnetic nanoparticles themselveseasily aggregatetogetherto formlargerparticleswithreducedspecic surface areas (52 m2/g), about twice lower than that oftheZn0.8Co0.2Fe2O4sample. Theabsorptionof MBsolutionsover ZnxCo1xFe2O4catalysts after reaching absorptiondesorptionequilibriuminthedarkreveals thelarger surfaceareaofthecatalyst, thestrongerabsorptionofMBmolecules(Figure S6). Since the photocatalytic reaction occurs mainly atthe interface between the photocatalyst and the target dyestu,theZn0.6Co0.4Fe2O4catalyst withlower surfaceareas cannotprovidesucientaccessibleactivesites. Correspondingly, lessMB moleculescan be adsorbed ontothe catalyst, leading to apoor photocatalytic activity. Further, temporal evolution of thespectral changestakingplaceduringthephotodegradationoftheMBmoleculemediatedbytheZn0.8Co0.2Fe2O4sampleisdisplayedinFigure9. Asexpected, agradual decreaseintheintensityof astrongabsorptionbandat 665nmfortheMBsolutionis observedduring the course of the photoassisteddegradation, indicatingthat thelargeconjugatedsystemofthe MB dye molecule has been destroyed.53,54Meanwhile, theother absorption at 292 and 245 nm related to thephenothiazine species decreases along withirradiationtime,implying that oxidative decomposition has occurred. To furtherinvestigate the photocatalytic activity of samples, the TOCexperiment wasperformed. After8hirradiation, theremovaleciency of TOC for Zn1xCoxFe2O4 (x = 0, 0.03, 0.1, 0.2, and0.4) catalysts is 39, 45, 50, 53, and 22%, respectively, implyingthat the degradation of MBover ZnCo ferrites probablyproducesalargeamountof carbondioxide. Theaboveresultfurther demonstrates that Zn0.8Co0.2Fe2O4 indeed exhibits goodphotocatalytic activity under visible light irradiation. ValenzuelagrouphaspreparedaZnFe2O4photocatalyst viaacoprecipi-tation method followed by calcining at 800 C for 8 h.55Due tothe high surface area (35.4 m2/g), the obtained ZnFe2O4 showshighphotoactivityinthephotodegradationof phenol. Wangandco-workershavesynthesizedZnFe2O4nanospheresviaatemplate-free solvothermal route.56Compared with theZnFe2O4nanoparticle, theZnFe2O4nanospheresexhibithighsurfacearea(51.8m2/g)andenhancedphotocatalyticactivityinthephotodegradationof dyeinwastewater. However, theusing of organic solvent during the synthesis process ofFigure 8. Photodegradation of MBmonitored as the normalizedconcentration change vs irradiation time under visible light irradiation:without catalyst (a); Zn1xCoxFe2O4 samples: (b) x = 0, (c) x = 0.03;(d) x = 0.1; (e) x = 0.2; and (f) x = 0.4.Figure 9. Absorption changes of MBsolution during the photo-degradationprocessovertheZn0.8Co0.2O4sampleundervisiblelightirradiation.Industrial & Engineering Chemistry Research Articledx.doi.org/10.1021/ie201933g | Ind. Eng. Chem. Res. 2012,51, 1363913647 13644ZnFe2O4limits its scale-up application. Therefore, theadvantage of our route is that ZnCo ferrites with highersurface areas and good photocatalytic performance for organicpollutants can be formed at a lower temperature without usingan expensive solvent, which exhibits the industrializationtendencies for practical applications.As showninFigure10, whenamagnet withtheexternalmagnetic eldof 0.15Tapproachesthedispersionof MBinwater at room temperature, Zn0.8Co0.2Fe2O4particles areattracted to the sidewall of the magnet within a relativelyshort period of about 5 s, and the dispersion becomes clear andtransparent(SupportingInformation, Movie). Itdemonstratesthat Zn0.8Co0.2Fe2O4 particles can be magnetized and enrichedby the external magnetic eld due to their paramagneticbehavior, although the sample is of small magnetization value atthe external magnetic eld. Such magnetic response ofZn0.8Co0.2Fe2O4particles is not surprising, and similarphenomenon has been found in other ZnFe2O4-basedcompositesreportedintheliterature.57,58Whilethemagneticeld is removed, the magnetization decays rapidly to zero, andZn0.8Co0.2Fe2O4 particles can be readily redispersed in aqueoussolution.In addition, after recycling for four times, the photocatalyticreactionrateofMBovertheZn0.8Co0.2Fe2O4sampleremainsalmostunchanged(FigureS7). Elemental analysisbyICP-ESrevealsthattheZnandColeachinglossesareonlyabout0.9and 0.2 wt.% for the Zn0.8Co0.2Fe2O4 sample after recycling forfour times, suggestive of the photostability of the ferrite sampleduring photodegradation.These results lead to the conclusionthat as-synthesized ZnCo ferrites in the present study are veryeective visible-light-induced photocatalysts for the degradationof MB molecules.At last, 2,4,6-trichlorophenol andsalicylicacidsodiumareused as colorless pollutants to investigate the catalyticperformance of the Zn0.8Co0.2Fe2O4catalyst. As shown inFigure 11, after 8 h irradiation, the degradation percentages for2,4,6-trichlorophenol and salicylic acid sodium reach about 81%and 90% over the Zn0.8Co0.2Fe2O4catalyst, indicative ofsuperior photocatalytic performance for the degradation ofcolorless pollutants to commercially used P25.4. CONCLUSIONSThisstudyprovidedafacilereductionoxidationapproachtofabricatevisible-light-inducedcobalt-dopedzincferritephoto-catalysts. WiththeincreaseinCocontent, thebandgapofZn1xCoxFe2O4(x = 0, 0.03, 0.1, 0.2) ferrites decreasesgradually, andthereducedbandgapof ferritesthusleadstoan enhanced photocatalytic activity for degradation of MBunder visiblelight irradiation. Especially, theZn0.8Co0.2Fe2O4sampleexhibits muchhigher photocatalytic activitythanthepure ZnFe2O4 sample. The method used in this study is foundto be of great advantage to prepare multicomponent magneticferrite materials possessing desirable visible-light photocatalyticactivity. It can be expected that such a new type ofphotocatalysts is promising for practical application in theeld of industrial visible-light-induced photodegradation oforganic pollutants, in terms of their intrinsic properties, such ashigh chemical stability and strong resistance to acid and alkali.ASSOCIATED CONTENT*S Supporting InformationXPS spectra of ZnFe2O4and Co substituted zinc ferrites,photodegradation of MB monitored as the normalizedconcentration change vs irradiation time, the absorption ofMB solution over the dierent sample after reachingabsorptiondesorptionequilibriuminthe absence of visiblelight irradiation, andpseudo-rst-orderkineticforthephoto-degradation of MB for the Zn0.8Co0.2Fe2O4sample afterFigure10. PhotographrepresentingthemagneticenrichmentoftheZn0.8Co0.2Fe2O4 sample from MB solution.Figure 11. Photocatalytic degradation of 2,4,6-trichlorophenol (A) and salicylic acid sodium (B) as a function of reaction time under visible light.Samples: without catalyst (a); P25 (b); and Zn0.8Co0.2Fe2O4 (c).Industrial & Engineering Chemistry Research Articledx.doi.org/10.1021/ie201933g | Ind. Eng. Chem. Res. 2012,51, 1363913647 13645recyclingfordierent times. Thismaterial isavailablefreeofcharge via the Internet at http://pubs.acs.org.AUTHOR INFORMATIONCorresponding Author*Phone: 8610-64451226. Fax: 8610-64425385. E-mail: lifeng@mail.buct.edu.cn.NotesThe authors declare no competingnancial interest.ACKNOWLEDGMENTSThis work is nancially supported by the National BasicResearchProgramofChina(GrantNo. 2011CBA00506)andthe National Natural Science Foundation of China.REFERENCES(1)Li, X.; Xiong, R.;Wei, G. EnhancedPhotocatalyticActivityforTitaniumDioxide by Co-Modifying with Silica and Fluorine. J.Hazard.Mater.2010, 175, 258.(2) Prevot, A. B.; Baiocchi, C.; Brussino, M. C.; Pramauro, E.;Savarino, P.; Augugliaro, V.; Marc, G.; Palmisano, L. PhotocatalyticDegradationofAcidBlue80inAqueousSolutionsContainingTiO2Suspensions.Environ.Sci.Technol.2001, 35,971.(3)Li, B.; Wang, Y. FacileSynthesisandEnhancedPhotocatalyticPerformance of Flower-like ZnO Hierarchical Microstructures. J. Phys.Chem.C 2010,114, 890.(4) Mhan, K. S. U.; Al-Shahry, M.; Ingler, W. B., Jr. EfficientPhotochemical Water Splitting by a Chemically Modified n-TiO2.Science 2002,297, 2243.(5) Song, S.; Tu, J.; He, Z.; Hong, F.; Liu, W.; Chen, J. Visible Light-Driven Iodine-Doped TitaniumDioxide Nanotubes Prepared byHydrothermal Process and Post-Calcination. Appl. Catal., A 2010, 378,169.(6) Li, Y.; Zhou, X.; Hu, X.; Zhao, X.; Fang, P. Formation of SurfaceComplex Leading to Efficient Visible Photocatalytic Activity andImprovement of Photostabiltyof ZnO. J. Phys. Chem. C2009, 113,16188.(7) Wang, R.; Xu, D.; Liu, J. B.; Li, K. W.; Wang, H. Preparation andPhotocatalytic Properties of CdS/La2Ti2O7Nanocomposites underVisible Light.Chem.Eng.J.2011,162, 455.(8) Choi, Y. W.; Kim, E. J.; Hahn, S. H. Photocatalytic Activity of Au-BufferedWO3ThinFilms Preparedby RFMagnetronSputtering.Chem.Eng.J.2010, 161,285.(9) Park, H.; Choi, W. Y. Visible Light and Fe(III)-mediatedDegradation of Acid Orange 7 in the Absence of H2O2. J. Photochem.Photobiol.,A 2003,159, 241.(10) Quickel, T. E.; Le, V. H.; Brezesinski, T.; Tolbert, S. H. On theCorrelation between Nanoscale Structure and Magnetic Properties inOrderedMesoporous Cobalt Ferrite(CoFe2O4) ThinFilms. NanoLett.2010, 10,2982.(11) Song, Q.; Zhang, Z. J. Shape Control and Associated MagneticProperties of Spinel Cobalt FerriteNanocrystals. J. Am. Chem. Soc.2004, 126, 6164.(12)Bao, N.; Shen, L.; Wang, Y.; Padhan, P.; Gupta, A. AFacileThermolysis Route to Monodisperse Ferrite Nanocrystals. J. Am.Chem.Soc.2007, 129,12374.(13) Bodnarchuk, M. I.; Kovalenko, M. V.; Pichler, S.; Fritz-Popovski,G.; Hesser, G.; Heiss, W. Large-Area Ordered Superlattices fromMagnetic Wustite/CobaltFerriteCore/ShellNanocrystalsby DoctorBlade Casting.ACS Nano 2010, 4, 423.(14) Casbeer, E.; Sharma, V. K.; Li, X. Z. Synthesis andPhotocatalytic Activity of Ferrites under Visible Light: AReview.Sep.Purif.Technol.2012, 87, 1.(15) Su, M. H.; He, C.; Sharma, V. K.; Asi, M. A.; Xia, D. H.; Li, X.Z.; Deng, H. Q.; Xiong, Y. Mesoporous Ferrite Nanoparticles:Synthesis, Characterization, andPhotocatalytic ActivitywithH2O2/Visible Light.J.Hazard.Mater.2012, 211212, 95.(16)Sivakumar, M.; Takami, T.; Ikuta, H.; Towata, A.; Yasui, K.;Tuziuti, T.; Kozuka, T.; Bhattacharya, D.; Iida, Y. Fabrication of ZincFerrite Nanocrystals by Sonochemical Emulsification and Evaporation:Observation of Magnetization and Its Relaxation at Low Temperature.J.Phys.Chem.B 2006, 110,15234.(17) Grasset, F.; Labhsetwar, N.; Li, D.; Park, D. C.; Saito, N.;Haneda, H.; Cador, O.; Roisnel, T.; Mornet, S.; Duguet, E.; Portier, J.;Etourneau., J. Synthesis and Magnetic Characterization of Zinc FerriteNanoparticles with Different Environments: Powder, ColloidalSolution, and Zinc FerriteSilica CoreShell Nanoparticles. Langmuir2002,18,8209.(18) Liu, F. F.; Li, X. Y.; Zhao, Q. D.; Hou, Y.; Quan, X.; Chen, G. H.Structural andPhotovoltaicProperties of HighlyOrderedZnFe2O4NanotubeArraysFabricatedbyaFacileSolGel TemplateMethod.Acta Mater.2009,57,2684.(19)Fan, G.; Gu, Z.; Yang, L.; Li, F. NanocrystallineZincFerritePhotocatalysts Formed Using the Colloid Mill and HydrothermalTechnique.Chem.Eng.J.2009, 155, 534.(20) Pileni, M. P. Magnetic Fluids: Fabrication, Magnetic Properties,and Organization of Nanocrystals.Adv.Funct.Mater.2001, 11, 323.(21)Cao, S. W.; Zhu, Y. J.; Cheng, G. F.; Huang, Y. H. ZnFe2O4Nanoparticles: Microwave-Hydrothermal IonicLiquidSynthesis andPhotocatalytic PropertyOver Phenol. J. Hazard. Mater. 2009, 171,431.(22)Cao, X. B.;Gu, L.;Lan, X. M.;Zhao, C.;Yao, D.;Sheng, W.Spinel ZnFe2O4 Nanoplates Embedded with Ag Clusters: Preparation,Characterization, and Photocatalytic Application. J. Mater. Chem. Phys.2007,106, 175.(23)Li, X.;Hou, Y.;Zhao, Q.;Wang, L. AGeneral, One-StepandTemplate-free Synthesis of Sphere-like Zinc Ferrite Nanostructureswith Enhanced Photocatalytic Activity for Dye Degradation. J. ColloidInterface Sci.2011, 358, 102.(24)Tsukada, M.; Abe, K.; Yonemochi, Y.; Ameyama, A.; Kamiya,H.; Kambara, S.; Moritomi, H.; Uehara, T. Dry Gas Cleaning in CoalGasification Systems for Fuel Cells Using Composite Sorbents. PowderTechnol.2008,180, 232.(25)Hochepied, J. F.;Bonville, P.;Pileni, M. P. NonstoichiometricZincFerriteNanocrystals: SynthesesandUnusual MagneticProper-ties.J.Phys.Chem.B 2000,104, 905.(26)Atif, M.;Hasanain, S. K.;Nadeem, M. MagnetizationofSolGel Prepared Zinc Ferrite Nanoparticles: Effects of InversionandParticle Size.Solid State Commun.2006, 138,416.(27) Yang, J. M.; Yen, F. S. Evolution of Intermediate Phases in theSynthesis of Zinc Ferrite Nanopowders Prepared by the TartratePrecursor Method.J.Alloys Compd.2008, 450,387.(28)Zhao, J. A.; Mi,L. W.; Hou, H.W.; Shi, X.J.;Fan, Y.T. ThePreparation of Zinc Ferrite Nanorods by Using Single FerrocenylComplex as Precursor.Mater.Lett.2007, 61,4196.(29) Widatallah, H. M.; Al-Omari, I. A.; Sives, F.; Sturla, M. B.;Stewart, S. J. DynamicMagneticBehaviorof Cluster-GlassZnFe2O4Nanosystem.J.Magn.Magn.Mater.2008, 320,e320.(30) Jean, M.; Nachbaur, V. Determination of Milling Parameters ToObtainMechanosynthesizedZnFe2O4. J. Alloys Compd. 2008, 454,432.(31)Xue, H.;Li, Z. H.;Wang, X. X.;Fu, X. Z. FacileSynthesisofNanocrystalline Zinc Ferrite via a Self-propagating CombustionMethod.Mater.Lett.2007, 61, 347.(32) Misra, R. D. K.; Gubbala, S.; Kale, A.; Egelhoff-Jr, W. F. AComparison of the Magnetic Characteristics of Nanocrystalline Nickel,Zinc, and Manganese Ferrites Synthesized by Reverse MicelleTechnique.Mater.Sci.Eng.,B 2004,111, 164.(33) Gabal, M. A.; El-Bellihi, A. A.; El-Bahnasawy, H. H. Non-isothermal Decomposition of Zinc OxalateIron(II) Oxalate Mixture.Mater.Chem.Phys.2003, 81,174.(34) Kundu, A.; Anand, S.; Verma, H. C. ACitrate Process ToSynthesizeNanocrystallineZincFerritefrom7to23nmCrystalliteSize.Powder Technol.2003, 132, 131.Industrial & Engineering Chemistry Research Articledx.doi.org/10.1021/ie201933g | Ind. Eng. Chem. Res. 2012,51, 1363913647 13646(35) Mouallem-Bahout, M.; Bertrand, S.; Pen a, O. Synthesis andCharacterization of Zn1xNixFe2O4Spinels Prepared by a CitratePrecursor.J.Solid State Chem.2005, 178, 1080.(36) Zhao, Y.; Li, F.; Zhang, R.; Evans, D. G.; Duan, X. Preparationof Layered Double-Hydroxide Nanomaterials with a UniformCrystalliteSizeUsingaNewMethodInvolvingSeparateNucleationand Aging Steps.Chem.Mater.2002,14,4286.(37) Ryu, J. R.; Kim, S. W.; Kang, K.; Park, C. R. Synthesis ofDiphenylalanine/Cobalt Oxide Hybrid Nanowires and Their Applica-tion to Energy Storage.ASC Nano 2010, 4, 159.(38)Tong, G. X.;Guan, J. G.;Xiao, Z. D.;Mou, F. Z.;Wang, W.;Yan, G.Q.In Situ Generated H2 Bubble-Engaged Assembly: A One-StepAppraochfor Shape-ControlledGrowth of Fe Nanoparticles.Chem.Mater.2008, 20, 3535.(39)Gu, Z. J.; Xiang, X.; Fan, G. L.; Li, F. FacileSynthesis andCharacterization of Cobalt Ferrite Nanocrystals via a SimpleReduction-Oxidation Route. J.Phys.Chem.C 2008,112, 18459.(40) Thimmaiah, S.; Rajamathi, M.; Singh, N.; Bera, P.; Meldrum, F.;Chandrasekhar, N.; Seshadri, R. ASolvothermal Route toCappedNanoparticles of -Fe2O3andCoFe2O4. J. Mater. Chem. 2001, 11,3215.(41) Altavilia, C.; Ciliberto, E. Decay Characterization of GlassyPigments: An XPS Investigation of Smalt Paint Layers. Appl. Phys. A:Mater.Sci.Process.2004, 79, 309.(42) Allen, G. C.; Hallam, K. R. Characterisationof the SpinelsMxCo1xFe2O4(M= Mn, Fe or Ni) Using X-ray PhotoelectronSpectroscopy.Appl.Surf.Sci.1996, 93, 25.(43)Yamashita, T.;Hayes, P. Analysisof XPSSpectraof Fe2+andFe3+Ions in Oxide Materials.Appl.Surf.Sci.2008, 254,2441.(44) Li, Q.; Xie, R. C.; Li, Y. W.; Mintz, E. A.; Shang, J. K. EnhancedVisible-Light-Induced Photocatalytic Disinfection of E. coli by Carbon-Sensitized Nitrogen-Doped TitaniumOxide. Environ. Sci. Technol.2007, 41, 5050.(45) Ng, C. H. B.; Fan, W. Y. Shape Evolution of Cu2ONanostructuresviaKineticandThermodynamicControlledGrowth.J.Phys.Chem.B 2006, 110, 20801.(46)Li, F.; Wang, H.; Wang, L.; Wang, J. MagneticPropertiesofZnFe2O4Nanoparticles ProducedbyaLow-temperatureSolid-StateReaction Method.J.Magn.Magn.Mater.2007, 309,295.(47)Blanco-Gutierrez, V.;Climent-Pascual, E.; Torralvo-Fernandez,M. J.; Saez-Puche, R.; Fernadez-Diaz, M. T. Neutron Diffraction Studyand Superparamagnetic Behavior of ZnFe2O4 Nanoparticles Obtainedwith Different Conditions.J.Solid State Chem.2011, 184, 1608.(48) Blanco-Gutie rrez, V.; Jime nez-Villacorta, F.; Bonville, P.;Torralvo-Ferna ndez, M. J.; Sa ez-Puche, R. X-ray AbsorptionSpec-troscopyandMo ssbauerSpectroscopyStudiesof SuperparamagneticZnFe2O4 Nanoparticles.J.Phys.Chem.C 2011, 115,1627.(49) Blanco-Gutie rrez, V.; Torralvo-Ferna ndez, M. J.; Sa ez-Puche, R.Magnetic Behavior of ZnFe2O4 Nanoparticles: Effects of a Solid Matrixand the Particle Size.J.Phys.Chem.C 2010, 114, 1789.(50) Stoner, E. C.; Wohlfarth, E. P. AMechanismof MagneticHysteresis in Heterogeneous Alloys.Trans.R.Soc.1948, A240, 599.(51) Brown, W. F. Thermal Fluctuations of A Single-DomainParticle.Phys.Rev.1963,130, 1677.(52) Song, Q.; Zhang, Z. J. Correlation between Spin-OrbitalCoupling and the Superparamagnetic Properties inMagnetite andCobalt Ferrite Spinel Nanocrystals. J. Phys. Chem. B 2006, 110, 11205.(53) Lakshmi, S.; Renganathan, R.; Fujita, S. Study on TiO2-Mediated Photocatalytic Degradation of Methylene Blue. J. Photochem.Photobiol.,A 1995,88,163.(54) Shu,X.; He,J.; Chen, D.Tailoring of Phase Composition andPhotoresponsiveProperties of Ti-ContainingNanocomposites fromLayered Precursor.J.Phys.Chem.C 2008,112, 4151.(55)Valenzuela, M. A.;Boshc, P.;Jime nez-Becerrill, J.;Quiroz, O.;Pa ez, A. I. Preparation, Characterization and Photocatalytic Activity ofZnO, Fe2O3 and ZnFe2O4. J. Photochem. Photobiol., A 2002, 148, 177.(56)Li, X. Y.;Hou, Y.;Zhao, Q. D.;Wang, L. Z. AGeneral, One-Step and Template-Free Synthesis of Sphere-Like Zinc FerriteNanostructures with Enhanced Photocatalytic Activity for DyeDegradation.J.Colloid Interface Sci.2011,358, 102.(57) Fu, Y. S.; Wang, X. Magnetically Separable ZnFe2O4-GrapheneCatalyst and its High Photocatalytic Performance under Visible LightIrradiation.Ind.Eng.Chem.Res.2011, 50, 7210.(58) Zhang, B.; Zhang, J.; Chen, F. Preparation and Characterizationof Magnetic TiO2/ZnFe2O4 Photocatalysts by a Sol-Gel Method. Res.Chem.Intermed.2008, 34, 375.Industrial & Engineering Chemistry Research Articledx.doi.org/10.1021/ie201933g | Ind. Eng. Chem. Res. 2012,51, 1363913647 13647